Process for cracking hydrocarbons



N. PECHTOLD ETAL PROCESS FOR CRACKING HYDROCARBONS Filed June 20. 1961 INVENTORS N/KOLAUS PECHT OLD RUDOLF W/MMER RUDOLF W/RTZ WERNER F/SCHER BY dun-E We: 1-

A TORNEYS March 15, 1966 United States Patent F 13 Claims. 61. 260679) The present invention provides a process for cracking hydrocarbons.

It is known to crack hydrocarbons by introducing them into a current of hot carrier gases and chilling them after a short time of reaction. The carrier gases are preferably produced by burning hydrogen, carbon monoxide and/ or hydrocarbons with oxygen or oxygencontaining gases. This process is particularly suitable for preparation of acetylene and ethylene from hydrocarbons. As by-products there are inter alia formed propylene and other olefins, carbon monoxide, carbon dioxide, methane, cracking oils and coke. The cracking oils and coke easily lead to the formation of deposits in the reactor which render a continuous operation difficult or even impossible. The quantity of coke depositing in the reactor is the greater the higher the depth of cracking, that is to say, the higher the yields of acetylene, since the consistency of the cracking oils becomes more and more solid as the yields of acetylene increase.

For this reason, for the preparation of acetylene and ethylene by thermal splitting there have hitherto preferably been used in practice hydrocarbons of low molecular weight since these hydrocarbons lead only to small deposits of coke. Starting products of this kind are, for example, propane, butane or light naphthene (upper limit of the boiling range between 100 and 150 C.). I

The deposition of coke in the reactor can be restrained to a certain degree by the additon of steam to the hot carrier gases. However, this measure suffices to ensure a continuous operation only in cases in which the depth of cracking is not too high and the tendency of the oils to form coke is not too strong. Moreover, the said measure involves economical disadvantages. Since the steam is heated by the hot carrier gases to the reaction temperature it uses a part of the energy which otherwise would be available for the splitting reactions. To the cost of the steam itself is added in this case the cost of the increased consumption of energy. The costs are not yet very important when the depth of cracking is moderate (temperature at the end of the reaction about 800 C.). The cost of the addition of steam becomes, however, a considerable economical factor and has to be kept as low as possible for the above reasons when the depth of cracking is high and reaction temperatures above 1000 C. have to be applied.

Now we have found that the preparation of principally acetylene and ethylene by mixing hydrocarbons with a current of hot carrier gases and cracking the hydrocarbons is favoured by rinsing the walls of the reactor with steam in such a manner that the whole inner wall of the reactor is covered with a veil of steam which mixes only partially with the carrier and cracking gases. In this way the above-described disadvantages are avoided to a large extent. Since the steam is conducted exclusively along the inner wall of the reactor the quantity of steam is the less important the larger the apparatus. Since, moreover, the steam is heated only partially to the final reaction temperature the quantity of combustion gas and oxygen which is required is not considerably increased.

3,240,836 Patented Mar. 15, 1966 The process according to the present invention may be applied for the cracking of all saturated hydrocarbons such as methane, ethane, butane, n-hexane, isooctane and cyclohexane and unsaturated hydrocarbons which with respect to the degree of saturation are similar to the saturated hydrocarbons, for example, n-butene, hexenes and dodecenes, and mixtures of these hydrocarbons. These mixtures may also contain hydrocarbons having a lower degree of saturation, for example, butadiene, vinyl acetylene and aromatic hydrocarbons such as benzene or toluene.

The process according to the invention is particularly siutable for the cracking of hydrocarbons which tend to deposit coke in the reactor. Among hydrocarbons of this kind are normal paraflins having a molecular weight above 200, iso-paraflins and naphtheneses having a molecular weight above 140, monoolefins having a molecular weight above and all compounds having a lower degree of saturation such as diolefins and acetylenes and all aromatic substances. Hydrocarbon mixtures containing larger portions of one or more of the aforesaid groups are also suitable for the cracking process according to the invention. The process of the invention is in particular concerned with petroleum distillates boiling completely or partially above C. However, the boiling point of the petroleum distillates is not decisive since the composition of a petroleum distillate and consequently its content of easily coking hydrocarbons depends on its origin and on its pretreatment.

The carrier gases to be used in the cracking process may be prepared by one of the known methods, for ex-. ample, by burning mixtures of hydrogen and hydrocarbons with oxygen in a cooled combustion chamber and by subsequently adding steam to the combustion gases. When this gas leaves the burner the hydrocarbon or the mixture of hydrocarbons to be cracked which will hereinafter be called starting material is added in an appro priate manner. In the following reactor it is cracked predominantly into acetylene and ethylene. The dimensions of the cylindrical or conical reactor are such that the residence time amounts to a few milliseconds. At the end of the reactor the reaction is interrupted in known manner by chilling with Water, Water and steam or water and oil.

At the upper end of the reactor steam is introduced along the wall, preferably in a direction parallel with that of the entering gases, in order to prevent any deposition of coke. In most cases it is important to add the steam at such a uniform rate that immediately after the entrance of the steam the wall of the reactor is covered at every place with a layer of steam of uniform thickness,

' and that the velocity at which it enters has the same di removed by the water gas reaction:

Because of its ability to react in this manner steam is preferred to all liquid rinsing agents such as water or oil,

which, besides, have the drawback of consuming a considerable portion of the energy from the carrier gas by partial evaporation Whereas the quantity of energy absorbed by the steam is comparatively small. As compared to other rinsing gases steam has the advantage that it can easily and at a low expense be removed by condensation so that the working up of the cracked gases is facilitated.

In general it is advantageous to add the rinsing steam not immediately after the addition of the oil but only as the gases enter the reaction zone. However, if strongly coking starting materials are used it may also be of advantage to subject already the walls of the mixing zone which is arranged before the reaction zone to a rinsing with steam.

It is favorable to keep the speed of the gas in the reactor above 100 meters per second and it is particularly advantageous to keep it Within the range of 200 to 500 meters per second, in order to remove mechanically any particles of coke and cracking oil which may deposit at the wall.

The quantity of steam which is required depends on the coking tendency of the hydrocarbon to be cracked. The quantity of steam is to be so large that upon entering the reactor the veil of steam has a thickness which is in general Within the range of 1 to 15 mm. and with particular advantage within the range of 2 to 7 mm. The layer of steam has to be the thicker the larger the formation of coke from the starting material. Besides, the veil of steam has to be the thicker the longer the reactor, so that even at the end of the reactor a layer of steam is present which has practically not been mixed with the cracking gas. In the case of very long reactors it may therefore be advantageous to introduce steam at several places in the way described above. Thus a new veil of steam is formed before the preceding one has become completely inefficient.

It is particularly advantageous to superheat the rinsing steam before it enters the reactor to a temperature within the range of 200 to 1000 (3., preferably within the range of 600 to 1000 C. By this measure the speed of the water gas reaction is increased and the deposition of coke in the reactor is prevented in a particularly efiicacious manner. Accordingly, the temperature to which the steam is preheated is the higher the stronger the coking tendency of the starting material. On the other hand, when the same starting material is used the quantity of steam required for the rinsing of the reactor may be reduced by increasing the temperature.

The deposition of coke at the wall of the reactor can be prevented in a particularly efiicacious manner by maintaining the temperature of the inner wall of the reactor at at least 700 C., preferably at a temperature within the range of 800 to 900 C. When the temperature of the wall is too low there is a danger that coke particles adhering to the wall become too cold to react quickly enough with the steam.

The invention will now be explained in greater detail, by way of example only, with reference to the accompanying drawings.

With reference to the drawings, FIG. 1 illustrates one mode of carrying out the process according to the inven tion and an apparatus which is suitable for this purpose. In a combustion chamber 1 the hot carrier gas to be used in the cracking process is produced by burning combustion gases 5 with oxygen 6 and adding steam 7. In the following mixing zone 2 the hydrocarbons 8 which are to be split are admixed in an appropriate manner with the carrier gas. The mixing zone 2 is so short that no considerable reaction can take place in it. The splitting reaction takes place in a cylindrical reactor 3 which is arranged after the mixing zone and rinsed with steam. The gases leaving the reactor are chilled at 4 in known manner, for example, With water. The rinsing steam 9 is.heated in an appropriate preheater 10 to a temperature within the range of 200 to 1000 C., preferably 600 to 1000 C.

The superheated steam issuing from the preheater 10 flows via an annular distributing chamber 11 through an annular slot 12 into the reaction chamber 3. In this way a uniform veil of steam is produced which covers the whole of the inner wall 13 of the reactor and which prevents any deposition of coke.

The wall of the reactor may be made of any heatresistant material, provided the material is resistant to steam of 1000 C. Metals or metal alloys which are resistant to high temperatures are particularly suitable. The inner wall of the reactor must not have uneven places for then the veil of steam would be interrupted. The inner wall of the reactor is advantageously polished to the highest degree of smoothness.

FIG. 2 illustrates another mode of carrying out the process of the invention. It differs from the mode of operating shown in FIG. 1 by the fact that before entering the reactor 3 the rinsing steam is conducted through a jacket 14 along the hot Wall 13 of the reactor and heated in this manner. By making use of the losses of heat of the reactor for heating the rinsing steam the economy of the process of the invention is considerably increased since the cost of the preheater and an additional heating agent are dispensed with.

If the heat dissipated by the reactor is not quite sufficient to heat the rinsing steam to the desired temperatureas may be the case when the steam enters at a very high temperaturethe superheating in the heating jacket according to FIG. 2 may be combined with a heating in a preheater according to FIG. 1, which has correspondingly smaller dimensions.

FIGS. 3 and 4 show two particular modes of realizing the process according to the invention.

FIG. 3 represents a cylindrical reactor rinsed with steam which is introduced in several stages during the reaction as is advantageous with very long reactors. The steam is introduced into the reactor in three stages via the pipes 1', 2', and 3. The steam enters the annular spaces 7', 8' and 9', passes through the annular slots 4, 5' and 6', respectively, and arrives at the three stages 11', 12' and 13, respectively, of the reaction chamber. The distances between the annular slots 4', 5' and 6' are such that a new quantity of steam is introduced before the veil of steam of the preceding stage has dissolved. This measure enables the whole of the inner wall of even very long recation tubes to be uninterruptedly covered wiltih a veil of steam which prevents any deposition of co e.

FIG. 4 shows a conical reactor which is rinsed with steam. The steam is introduced via the tube 1" into the annular distributing chamber 2 and conducted-advantageously in a direction parallel to that of the wall 5"via the annular slot 3" into the reaction chamber 6". A veil of steam forms along the wall 5" of the reactor which prevents the cracking and carrier gases leaving the mixing chamber 4" from coming into contact with the wall of the reactor. The reactor shown in this figure is particularly suitable for the cracking of oils having a strong tendency to form coke since by conducting the veil of steam conically the steam is restrained to a larger extent from mixing with the splitting gases than in the reactors which have been described above. The angle of aperture of the cone is within the range of 10 to 30, preferably 15 to 25.

The following example serves to illustrate the invention but it is not intended to limit it thereto,

Example In a combustion chamber 230 cubic meters (measured at N.T.P.) of hydrogen were burnt with cubic meters (at N.T.P.) of oxygen and mixed with 85 kilograms of steam. In a mixing zone kilograms of a petroleum distillate boiling between 40 and 360 C. which had a diameter of 68 mm. and which was surrounded with a jacket (according to FIG. 2) through which the rinsing steam flowed. The rinsing steam (110 kilograms) entered the reactor in a direction parallel to that of the carrier and splitting gases through an annular slot having a width of 4 mm. When the steam entered the reactor it had a temperature of about 600 C. At the outlet of the reactor the hot gases were chilled by the injection of water. A gas was obtained which had the following composition:

Percent by volume In the manner in which this experiment was carried out no coke had deposited in the reactor after 200 hours of reaction. When the experiment was repeated and the reactor which was rinsed with steam was replaced by a reactor having a diameter of 60 mm. and which was not rinsed with steam the process had to be stopped after 8 hours because of the strong deposition of coke in the reactor.

We claim:

1. A process for manufacturing acetylene and ethylene by thermally cracking hydrocarbons which comprises passing a mixture of said hydrocarbons and a hot carrier gas through a reaction zone and introducing into said reaction zone an annular current of steam surrounding said mixture and adjacent the walls forming said reaction zone, said current of steam having the same direction and at least the same velocity as said mixture.

2. The process of claim 1 wherein the velocity of the current of steam is 1 to 1.5 times that of the mixture.

3. The process of claim 1 wherein the velocity of the mixture is more than meters per second.

4. The process of claim 1 wherein the velocity of the mixture is in the range between 200 and 500 meters per second.

5. The process of claim 1 wherein the mixture is surrounded by a current of steam forming a veil of I to 15 mm. thickness.

6. The process of claim 1 wherein the mixture is surrounded by a current of steam forming a veil of 2 to 7 mm. thickness.

7. The process of claim 1 wherein the steam is superheated steam.

8. The process of claim 7 wherein the steam is superheated to a temperature between 600 and 1000 C.

9. The process of claim 7 wherein the steam is superheated by conducting it along the outer wall of the reaction zone.

10. The process of claim 1 wherein the inner wall of the reaction zone has a temperature about 700 C.

11. The process of claim 1 wherein the reaction zone is conically shaped.

12. The process of claim 11 wherein the angle of the cone forming the reaction zone is in the range between 10 and 30.

13. The process of claim 5 wherein the steam is introduced at several places along the inner wall of the reaction zone in such intervals that the veil of steam is not interrupted.

References Cited by the Examiner UNITED STATES PATENTS 2,877,279 3/ 1959 Fowler et a1 260683 2,912,475 11/1959 Krause et al 260679 2,941,021 6/ 1960 Krause et al 260679 2,97 8,5 21 4/1961 Braconier 260679 3,005,857 10/ 1961 Steinhofer et a1 260679 3,019,271 1/1962 Braconier et al 260679 3,055,957 9/ 1962 Braconier et al 260679 3,069,248 12/ 1962 Braconier et al 260679 3,073,875 1/ 1963 Braconier et al 260679 ALPHONSO D. SULLIVAN, Primary Examiner. 

1. A PROCESS FOR MANUFACTURING ACETYLENE AND ETHYLENE BY THERMALLY CRACKING HYDROCARBONS WHICH COMPRISES PASSING A MIXTURE OF SAID HYDROCARBONS AND A HOT CARRIER GAS THROUGH A REACTION ZONE AND INTRODUCING INTO SAID REACTION ZONE AN ANNULAR CURRENT OF STEAM SURROUNDING SAID MIXTURE AND ADJACENT THE WALLS FORMING SAID REACTION ZONE, SAID CURRENT OF STEAM HAVING THE SAME DIRECTION AND AT LEAST THE SAME VELOCITY AS SAID MIXTURE. 